On his way out the door, Nanny Bloomberg strikes again: New York City is on the cusp of banning anyone under 21 from buying cigarettes or e-cigarettes.

On Thursday, the City Council passed a bill hiking the legal age to buy cigarettes, and the law will come into effect six months after New York City mayor Michael Bloomberg signs it. “This is going to have an important effect reducing young people from starting to smoke. . . . This is literally a piece of legislation that will save lives,” said Christine Quinn, city-council speaker and recent unsuccessful mayoral candidate.

There’s no doubt smoking is unhealthy (although reflect for a moment on the fact that New York teens too young to smoke can legally obtain an abortion, without parental consent). But it’s no sure thing that the new ban will be effective. Just consider what 16-year-old New Yorker Nicole Spencer told the New York Times (note that she was so blasé about her underage smoking that she gave her full name and allowed the Times to photograph her): “I buy them off people or I bum them off people,” Spencer said while holding a cigarette. She started when she was 13, and estimated to the Times that half her high-school friends smoked as well. Does the City Council really think that, if teens five years below the current legal age are smoking, raising the legal age by three years is going to change their behavior?

The data back up Spencer’s experience. According to the American Lung Association, “almost 70 percent of adult smokers began smoking before they turned 18.” “Most smokers try their first cigarette around the age of 11, and many are addicted by the time they turn 14,” the group explains. The Centers for Disease Control warns, “Each day in the United States, nearly 4,000 people younger than 18 years of age smoke their first cigarette, and an estimated 1,000 youth in that age group become new daily cigarette smokers.” Yet federal law bans anyone younger than 18 from buying cigarettes. Clearly, teens have found ways — whether bumming off friends or buying from their elders — to obtain cigarettes despite current law.

The evidence on underage drinking suggests that raising the minimum age for buying cigarettes won’t change that dynamic. Just as teens have found ways to access alcohol, so they will devise ways to obtain cigarettes. “In 2011 the National Survey on Drug Use and Health reported that 25 percent of youth aged 12 to 20 years drink alcohol and 16 percent reported binge drinking,” notes the CDC website.

Putting aside the new ban’s lack of effectiveness, let’s consider why anti-smoking advocates in New York City are content to stop at 21. Why not raise the age to 26? After all, as the American Cancer Society notes, “According to the 2012 Surgeon General’s Report, very few people start smoking after age 25. Nearly 9 out of 10 adult smokers started by age 18, and 99 percent started by age 26.” Maybe, in an era where Obamacare ensures that young adults can be on their parent’s health-care plans until age 26, 26 should become the new 18.

There’s even some scientific evidence to support the idea. A 2010 New York Times piece on “emerging adulthood” reported on “a longitudinal study of brain development sponsored by the National Institute of Mental Health, which started following nearly 5,000 children at ages 3 to 16” and found “the children’s brains were not fully mature until at least 25.”

Yet most Americans would rightly bristle at the notion that no one is mature enough to make his own decisions on alcohol, smoking, voting, and joining the military until age 26. And that’s for good reason: Many Americans have to act maturely and responsibly in many or all situations before they hit 26. Even with brains perhaps not fully developed, older teens and twentysomethings are capable of making rational decisions while taking long-term consequences into account.

A key part of maturity is having the opportunity to make your own decisions, right or wrong. Smoking, like eating too much junk food or biking without a helmet, isn’t a great life choice. But unless we want a nation of permanent adolescents, it’s a choice that should be up to Americans, 18-year-olds and others, not to their government officials.